MINOR: applet: Add a function to finalize frontend appctx startup

appctx_finalize_startup() may be used to finalize the frontend appctx
startup. It is responsible to create the appctx's session and the frontend
conn-stream. On error, it is the caller responsibility to release the
appctx. However, the session is released if it was created. On success, if
an error is encountered in the caller function, the stream must be released
instead of the appctx.

This function should ease the init stage when new appctx is created.
diff --git a/include/haproxy/applet.h b/include/haproxy/applet.h
index bef5aa5..b9c581d 100644
--- a/include/haproxy/applet.h
+++ b/include/haproxy/applet.h
@@ -41,6 +41,7 @@
 void appctx_shut(struct appctx *appctx);
 
 struct appctx *appctx_new(struct applet *applet, struct cs_endpoint *endp);
+int appctx_finalize_startup(struct appctx *appctx, struct proxy *px, struct buffer *input);
 
 /* Helper function to call .init applet callback function, if it exists. Returns 0
  * on success and -1 on error.
diff --git a/src/applet.c b/src/applet.c
index b7711b1..4bb345b 100644
--- a/src/applet.c
+++ b/src/applet.c
@@ -74,6 +74,32 @@
 	return NULL;
 }
 
+/* Finalize the frontend appctx startup. It must not be called for a backend
+ * appctx. This function is responsible to create the appctx's session and the
+ * frontend conn-stream. By transitivity, the stream is also created.
+ *
+ * It returns 0 on success and -1 on error. In this case, it is the caller
+ * responsibility to release the appctx. However, the session is released if it
+ * was created. On success, if an error is encountered in the caller function,
+ * the stream must be released instead of the appctx.
+ */
+int appctx_finalize_startup(struct appctx *appctx, struct proxy *px, struct buffer *input)
+{
+	struct session *sess;
+
+	BUG_ON(appctx->sess || !(appctx->endp->flags & CS_EP_ORPHAN));
+
+	sess = session_new(px, NULL, &appctx->obj_type);
+	if (!sess)
+		return -1;
+	if (!cs_new_from_endp(appctx->endp, sess, input)) {
+		session_free(sess);
+		return -1;
+	}
+	appctx->sess = sess;
+	return 0;
+}
+
 /* reserves a command context of at least <size> bytes in the <appctx>, for
  * use by a CLI command or any regular applet. The pointer to this context is
  * stored in ctx.svcctx and is returned. The caller doesn't need to release